Many partial knee joint replacement prostheses have proved satisfactory in the treatment of patients with severe tibial femoral knee joint disease. Unfortunately, these prostheses although highly satisfactory for the treatment of patients with arthritis involving the tibial femoral portion of the knee joint do not treat or affect the relationship between the patella and the anterior articular surface of the femur. Patients after knee joint replacement arthrosplasty have frequently continued to complain of patello-femoral joint symptoms -- crepitus and pain. This widespread problem of patello-femoral arthritis symptomatic after knee joint arthro plasty has been recognized by many, and attempts have been made to provide prostheses or prosthetic modifications to treat this deficiency. Bechtol, Ewald, and Walker, among others have designed and produced prosthetic components to address this problem. Most of these prostheses rely on provision of an extension of the usual femoral bearing surfaces anteriorly and superiorly on the femur. These replace the femoral portion of the patello-femoral articular bearing surface. A depression or groove is provided in the mid-section of this extension to enhance patellar tracking. The patella, or "knee cap" is customarily resurfaced with an ultra high molecular weight polyethylene inlayed insert. Several years previously the same problem was approached by Doctor McKeever with the production of a metallic resurfacing prosthesis for the articular bearing portion of the patella. It did nothing to replace the diseased portion of the distal femur. More recently, Ewald developed an ultra high molecular weight polyethylene patellar insert, again failing to replace or otherwise substitute for the damaged anterior and superior patello-femoral portions of the femur. The results were unsatisfactory. Total patellectomy, the complete removal of the patella, or "knee cap", has been employed for many years to treat patello-femoral knee joint arthritis. Present efforts toward developing a patello-femoral prosthesis, patello-femoral modifications of present prostheses, and recently reports in the literature document the relatively unsatisfactory effect of total patellectomy.
All presently known designs for prosthetic replacement of patello-femoral joint destruction rely upon a point, or at best a line contact between the patellar bearing surfaces and the femoral bearing surfaces. A very high contact stress between the patella and femur under normal conditions and activities has been documented. This very small area of contact of presently available prostheses, the demonstrated large force magnitudes, and the resulting high bearing surface contact stresses cannot but be detrimental to the satisfactory performance of these prosthetic joints. This is expected to cause accelerated wear on the components and the production of considerable wear debris, which may be damaging to the patient's tissue and may contribute to loosening of the components.
Presently available prosthetic replacements for the patella of ultra high molecular weight polyethylene are cemented using methylmethacrylate into the remaining patellar trabecular bone. Considering the high joint contact stresses, and the high magnitudes of force bearing on these prosthetic knee caps, one would expect elastic and plastic deformations along the compression force lines with a resulting spreading in transverse dimensions of the plastic component. This effect resulting from a Poisson's ratio situation would be expected to generate high shear stress levels at the polyethylene-methacrylate interface and possibly at the methacrylate-bone interface. This could result in loosening.
A single metal resurfacing prosthesis for the patella has not been demonstrated to be of a long term value after considerable numbers of patients have been treated after many years of follow-up. The prosthesis is rarely used today. The ultra high molecular weight polyethylene patellar replacement caused severe wear of the distal femur, subsequent wear of the prothesis, and results in clinical failure.
There is a very significant loss of extension strength with patellectomy. This particularly affects the patients in stair climbing and is also specifically deleterious to the treatment of elderly patients with arthritis. These patients very frequently have extreme weakness of the quadriceps muscles in association with obesity. They cannot tolerate this weakening effect. An object of this invention is to provide a suitable knee replacement prosthesis which overcomes such problems.